Japanese non-examined patent publication H01-99714 discloses a circular saw for cutting a workpiece. This known circular saw is capable of varying the cutting speed of a saw blade according to the cutting area of a workpiece or other similar factors. In designing a circular saw of this type for cutting a workpiece, however, it is required to provide a technique for allowing smooth control of the cutting speed of a saw blade in response to actual cutting operation in order to improve smoothness of the cutting operation.